In the field of mobile communications, technologies enabling handover between radio access networks or mobile communication systems using the same or different radio access technologies (RATs) have been proposed. Such technologies, for example, enable user equipment, after completing a call in a radio access network, to enter a standby mode in a different radio access network. This in turn makes it possible to distribute the load of a radio access network to other radio access networks when the radio access network becomes congested and thereby makes it possible to increase the total capacity and throughput of the radio access networks.
FIG. 1 shows handover between radio access networks and mobile communication systems using the same or different RATs. In this example, a third generation system (3G-RAT system) including radio access networks using a first frequency f1 and a second frequency f2 and a second generation system (2G-RAT system) using a third frequency f3 coexist in the same geographical area. Examples of the 2G-RAT system include GSM and PDC systems. In FIG. 1, when user equipment UE completes a call using the first frequency f1, the user equipment UE enters a standby mode in the second frequency f2 or the 2G-RAT system.
FIG. 2 shows handover between systems using different RATs. In FIG. 2, user equipment UE switches from an active mode to an idle mode after completing a call. When the user equipment UE completes a call (S1), a base station of a serving cell (in this example, a base station eNB in the 3G-RAT system) sends a radio resource control (RRC) release signal indicating release of a connection to the user equipment UE (S2). In response to the RRC release signal, the user equipment UE returns an RRC release complete signal (S3). As a result, the connection is released. When the user equipment UE is to stand by in a different frequency or a different RAT system, the user equipment UE searches for a destination cell (performs a cell search) in the different frequency or the different RAT system. An instruction or redirection information indicating the different frequency or the different RAT system in which the user equipment UE is to perform the cell search is sent from the base station together with the RRC release signal or at a different timing. When the destination cell is determined, the user equipment UE receives, for example, broadcast information from the destination cell and sends a location area update request (S5). As a result, the user equipment UE is registered in a location area of the different RAT system (S6). The location area may also be called a tracking area (TA) or a routing area (RA). Then, the user equipment UE intermittently monitors a paging channel in the destination cell (in this example, a cell in a GSM system) and waits for an incoming call (S7). Technologies relating to handover between networks or systems using the same or different RATs are described, for example, in 3GPP TS25.331 and 3GPP TS25.304.